Micropositioning actuators are used for precision movement of optical and mechanical devices small distances, i.e. micrometers (.mu.m), with accuracies comparable to the wave length of light at rates up to a few kilohertz. Such micropositioning actuators are commercially available constructed of either lead zirconium titanate or lead magnesium niobate. In either instance, the actuators are formed using a crystal stack which comprises thin plates of the respective crystals interleaved between thin metal electrodes. Alternate metal electrodes are electrically connected together so that the electrodes on opposite sides of each crystal plate are oppositely charged, when a voltage is applied to the electrodes. Such an applied voltage on the electrodes causes the crystal layers to expand or swell. When a series of such crystal layers and interleaved electrodes are bonded together into a stack and a voltage is applied to the electrodes, the stack will expand along the axis normal to the planes of the stack layers and then contract, i.e., move in the opposite direction, when the voltage is removed. The precise movement which results from applying and removing a voltage to and from such a crystal stack may be used, for example, to deform mirrors to correct optical wavefronts (adaptive optics).
Actuators made with lead magnesium niobate crystal stacks are more desirable than actuators made with lead zirconium titanate stacks where stability (lack of sagging) is important. However, lead magnesium niobate crystals are fragile. When the interleaved crystal/electrode layers comprising the actuator are all bonded together and the actuator is firmly fixed, i.e., bonded, to the object to be moved, e.g., a deformable mirror, the forces exerted on the crystal stack during contraction can cause damage to the crystal layers as each crystal layer exerts a contractive or pulling force on its opposite surfaces against the electrodes to which these crystal surfaces are respectively bonded. Such contractive forces exerted by each crystal layer against adjacent metal electrode layers to which the crystal is bonded on its opposite sides can tend to pull some of the crystals in the stack apart if the forces are great enough.
Since one of the intended uses of such an actuator is to deform a mirror, wherein the contractive forces may be as high as 200 Newtons, such potential damage to lead magnesium niobate actuators has limited the use of actuators made from this material for such purposes.
It would, therefore, be desirable to provide a lead magnesium niobate actuator wherein such contractive forces do not cause damage to the lead magnesium niobate crystal stack.